US Attorney General Eric H. Holder Jr. has authorized federal prosecutors to seek the death penalty for Dzhokhar Tsarnaev, the young man accused in the Boston Marathon bombings in April that killed three people, injured more than 260 others, and sent a wave of shock and fear through the region.

“After consideration of the relevant facts, the applicable regulations and the submissions made by the defendant’s counsel, I have determined that the United States will seek the death penalty in this matter. The nature of the conduct at issue and the resultant harm compel this decision,” Holder said in a statement.

“We support this decision and the trial team is prepared to move forward with the prosecution,” Boston US Attorney Carmen M. Ortiz said in a statement.

Miriam Conrad, who is leading Tsarnaev’s defense team, had no comment.

US District Court George A. O’Toole Jr. had given prosecutors until Friday to make the decision. With the authorization, the US attorney’s office can move forward in requesting a trial date. The death penalty can also now be a factor in any plea negotiations.

Tsarnaev, 20, faces 30 federal charges for setting off the April 15 bombs that plunged the region into terror for five days, until his arrest in Watertown. His older brother and alleged accomplice, Tamerlan, 26, was killed in a firefight with police.

Krystle Campbell, 29, Lingzi Lu, 23, and Martin Richard, 8, were killed in the blasts. Dzhokhar Tsarnaev also faces state charges in the fatal shooting of MIT Police Officer Sean Collier, who authorities say the brothers murdered as they tried to flee the area.

Prosecutors said in today’s filing with the court they would seek the death penalty for 16 of the charges Tsarnaev faces, and they would prove the following factors, as required by law: that the killings and injuries were intentional, that Tsarnaev willingly took part in the acts that resulted in death, and that he knew they could end in death.

The prosecutors also cited other factors that would allow for the death penalty: that death occurred during the commission of another crime, that the crime itself created a grave risk of death, that there was planning involved, that there were multiple killings and vulnerable victims, and that there was a “heinous, cruel and depraved manner of committing the offense.”

Prosecutors also cited several arguments that they weren’t required to make by law, including “betrayal of the United States,” and the selection of the Marathon as the site of the bombings.

Tsarnaev “enjoyed the freedoms of a United States citizen [then] betrayed his allegiance to the United States by killing and maiming people in the United States,” prosecutors said, adding that Tsarnaev had “demonstrated a lack of remorse.”

Prosecutors in federal cases where the death penalty is a possibility are required to decide at the outset whether to seek it. As attorney general, Holder held the ultimate authority to make the decision.

Prosecutors from Ortiz’s office and defense lawyers were allowed to file confidential recommendations to Holder in recent months, stating their arguments for and against the death penalty. Ortiz has not said what her recommendation was.

Ortiz said in her statement today that “the process by which this decision was made is confidential, and I will not comment further about that process other than to say that it entailed a careful and detailed consideration of the particular facts and circumstances of this case.”

Judge O’Toole has appointed four defense attorneys to represent Tsarnaev, including Judy Clarke, who specializes in death penalty cases.

If Tsarnaev is convicted, prosecutors would still have to present their arguments for the death penalty to a jury in a separate sentencing trial. Assistant US Attorney William D. Weinreb has said in previous court hearings that a trial could last three months, and that the separate sentencing trial could last an additional two months.

Jarrod Clowery, a 36-year-old carpenter from Milville whose legs were badly burned and struck by shrapnel during the bombings, said Holder’s decision had no emotional effect on him.

He never talks about the Tsarnaev brothers, something that happened naturally as he recovered from his injuries — not out of a conscious choice to avoid the subject.

“I’m moving on with my life,” he said. “It has no bearing on my life whatsoever … I don’t even think about the trial or anything like that. [The attackers] were tried and convicted by a power higher than us the moment they did what they did.”

A spokesman for the family of victim Martin Richard said they would have no comment on the prosecution decision.

Governor Deval Patrick said, “One way or another, based on the evidence, Tsarnaev will die in prison.”

“In each milestone of this case — today’s announcement, the trial and every other significant step in the justice process — the people hurt by the Marathon bombings and the rest of us so shocked by it will relive that tragedy. The best we can do is remind each other that we are a stronger Commonwealth than ever, and that nothing can break that spirit,” Patrick said in a statement.

Carol Rose, executive director of the American Civil Liberties Union of Massachusetts, said in a statement that her organization was disappointed in Holder’s decision and that it opposes the death penalty “because it is discriminatory and arbitrary, and inherently violates the Constitutional ban against cruel and unusual punishment.”

Rose noted that Massachusetts does not have a death penalty under state law. She pointed out how the community rallied around the slogan, “Boston Strong,” and said “that means not letting terrorists or anyone else shake us from staying true to our values.”

Holder’s decision means the federal court system in Massachusetts will have two pending death penalty cases. The United States is also seeking the death penalty for Gary Lee Sampson, who carjacked and killed three people in the same week in July 2001. A federal jury agreed to issue the death sentence for Sampson in 2003, but a federal judge vacated that decision in 2011, after finding that one of the jurors withheld information about herself during a screening process. An appeals court upheld the decision, and prosecutors are seeking the death penalty again.

Federal prosecutors have sought the death penalty two other times in Massachusetts. In the case of Darryl Green and Branden Morris, two Dorchester gang members, federal prosecutors ultimately dropped the charges and the men were tried in state court. In the case of Kirsten Gilbert, the former nurse who was convicted in 2001 of administering lethal injections to patients, a jury chose a life sentence rather than the death penalty.

Since 1988, when federal death penalty laws were first passed, juries nationwide have had to choose a punishment for 282 defendants. They’ve chosen death in 73 instances, or 34 percent of the time, according to Federal Death Penalty Resource Counsel, which tracked the figures through October.

A Globe poll in September showed that 57 percent of Massachusetts respondents supported a life sentence for Tsarnaev, compared with 33 percent who favored the death penalty. The Boston Bar Association made a public declaration earlier this month in opposition of the death penalty, saying it only offered the “illusion of ultimate punishment.” The group noted that death penalties, even when granted, are rarely carried out.

Since the federal laws were enacted in 1988, only three people have been executed, according to Federal Death Penalty Resource Counsel.

David P. Hoose, a Northhampton-based lawyer who handles death penalty cases, said that while the federal government has authorized fewer capital cases in recent years, he expected it would seek the death penalty in this case, noting the extent of the bombings, the number of injuries and deaths, and the nature of overall nature of the terrorist attack.

“If they ever want to maintain any credibility that there is a federal death penalty and they’re going to use it, of course, they’re going to authorize it in this case,” he said. “They’re not ready to declare a de facto moratorium on the death penalty. If they didn’t approve this one, that’s what they’d be doing.”

Before Holder’s decision was announced, Liz Norden of Stoneham, whose two sons each lost a leg in the Marathon blasts, said she wanted prosecutors to seek the death penalty. She said all of the victims were given an opportunity last year to give their input on whether prosecutors should seek the penalty.

“My life – my kids’ lives – have been changed forever,” said Norden, who was speaking for herself and not her sons, J.P. and Paul. She said she has always been a supporter of the death penalty for particularly heinous crimes, and she will be disappointed if the government ultimately allows a plea bargain enabling Tsarnaev to get life without parole. She sees no reason the government should waste money keeping him alive in prison for decades before he dies.

Norden added, though, that neither of her sons wanted to take part in the discussion, and have consistently chosen to be silent on the subject.

“They don’t haver time, they don’t even think about it,” the mother said. “They’re focused on their recovery.”

The attorney for Katherine Tsarnaeva, as her name is spelled in court records, said she was stopped for speeding in Franklin in August.

Tsarnaeva, who also goes by the name Katherine Russell, was charged with operating after suspension and that charge was dismissed after she paid $150 in court costs.

“There was a time, of course, in her life, as you know, when things were complicated to say the least,” said her attorney John St. Cyr. “And so it was very difficult for her to be able to get to where she needed to go to pick up her mail — in Cambridge particularly.”

Tsarnaeva has not talked publicly since her husband was killed in a shootout with police in Watertown days after the bombings. Leaving court, she did not answer any questions. She only thanked her attorney for representing her before she got into a car driven by her sister.

“She’s a very bright and a very pleasant girl,” her attorney said. “I’m surprised with all that she’s been through that she seems to be upbeat and very easy to talk to.”